While working at Harvard and Mass General during the 1960s he studied many melanomas. In 1966 he and the dermatologist
Thomas B. Fitzpatrick created the first Pigmented Lesion Clinic in the United States. He went on to create Pigmented Lesion Clinics at Temple University and the University of Pennsylvania. He made several seminal contributions to the understanding of melanoma. '''''"Clark's Classification" of melanoma''''' Together with Martin Mihm, Clark described histogenic types of melanoma which differ in their epidemiology, as well as their clinical and histological appearance. The four major categories of Lentigo Maligna Melanoma, Superficial Spreading Melanoma, Nodular Melanoma, and Acral Lentiginous Melanoma loosely correspond to more recent molecular classification of the disease. '''''"Clark's level" and melanoma prognosis''''' Clark correlated the microscopic appearance of the tumor with careful followup of the progression of the tumor and the patient's outcome. From these studies he developed a five-part scale, based on the depth of penetration of the lesion from the epidermis into the dermis and down to the subcutaneous tissue, which can be used to predict the likely progression of the tumor and the prognosis for the patient. While largely superseded by other prognostic attributes, pathologists evaluating a melanoma still classify it according to its
Clark's level, and treating physicians consider that information to choose the appropriate treatment. Clark also helped to develop criteria for recognizing the early signs of a melanoma, important in promoting early diagnosis and treatment. He was a strong a proponent of patient education and gave many public lectures teaching people how to recognize the warning signs of a possible melanoma. ==Recognition==